A proteinaceous inhibitor of the digestive enzyme alpha-amylase (1,4-a-D-Glucan glucanohydrolase; EC 188.8.131.52) has been isolated and purified from kidney beans Phaseolus vulgaris. It is a glycoprotein and it specifically inhibits corresponding plant, bacterial and fungal enzymes. Maximum inhibitory activity, against alpha-amylase, occurs at 37° C and pH 5.5. The inhibitor has a molecular weight of 45,000-50,000 (Marshall, JJ and Lauda C.M., J. Biol. Chem., 250, 8030, 1975). Similar alpha-amylase inhibitors also occur in wheat, rye and some other beans (Marshall, J.J., Am. Chem. Soc. Symposium, Series 15, 266, 1975).